Commissioning and implementation of a stereotactic conformal radiotherapy technique using a general-purpose planning...

The purpose of this paper is to report on commissioning and clinical implementation of a customized system for pediatric stereotactic conformal radiotherapy (SCRT). The system is based on the Pinnacle treatment-planning system and its interfaces with other equipment: (1) Beam models were optimized for our compact blocking system and a new LINAC. (2) Three CT-to-density conversion tables were evaluated, one using tabulated data for a commercial phantom, the second including additional points from the manufacturer's data for the inserts in an in-house phantom, and the third using measured densities for the in-house phantom materials combined with tabulated data for the commercial phantom. (3) Blocks were transferred to a computerized block cutter using in-house software that extracted the block shape from the export file and custom-fitted the additional necessary shapes. (4) In the absence of a DICOM RT Image link, a method based on screen data capture was used to export digitally reconstructed radiographs (DRRs) to two portal imaging systems for treatment verification. Lens shielding by multileaf collimation in the anterior-posterior isocenter verification field was investigated. (1) Computed dose distributions using the beam models agreed with measurements well within published acceptability criteria. A difference of up to 1.0 mm was measured between the beam's eye views of aperture blocks and computed 50% isodose contours for a 2 x 2 x 2 mm dose calculation grid. (2) The third table, which included measured densities, improved the accuracy of the calculated isocenter dose by up to 0.5% in typical patient SCRT treatments and up to 1.0% in a phantom with 5-cm diameter inhomogeneity inserts. (3) The block export and customization process was shown to introduce no additional uncertainty. A 1-mm block production uncertainty was measured using film dosimetry on six blocks. (4) The DRR transfer method did not introduce uncertainty into the process. Verification field shielding reduced lens dose by 12 to 15 times. In conclusion, this customized system for planning and verification of pediatric SCRT provides a high level of precision as well as reasonable practical efficiency.